Method of case hardening



. be maintained below about 20%.

Patented Aug. 30, 1938 UNITED STATES 'PATET METHOD OF CASE HARDENING No Drawing. Application February 10, 1937,

Serial No. 125,157

8 Claims.

This invention relates to case-hardening steels. One object is to provide an improved case-hardened steel product. Another object is to provide an improved method of hardening case-carburized steels. Still another object is to provide a steel composition adapted to'be case hardened by the practice of the thermal hardening method described and claimed in Bain et a1. Patent No. 1,924,099 issued August 29, 1933. Other objects and advantages will be apparent as the invention is more fully hereinafter disclosed.

Heretofore in the art the case hardening of steels has been accomplished in the following general manner:

The steel object after fabrication to its desired final size, shape and configuration is heated under surface carburizing conditions-to temperatures above the critical range and maintained at this temperature for a time interval adapted to produce the desired depth to the carburized area. The thus carburized object is then rapidly cooled, as by quenching, to near atmospheric temperatures to harden the carburized area and subsequently is tempered.

In the practice of this quench and temper" method of hardening the case-carburized area it has been found that in order to avoid the introduction of deleterious magnitudes of internal stresses and strains within the interior core metal of the object the carbon content of the core must sate for loss in strength incident to this low carbon in the core metal it heretofore in the art has been customary to add to the steel composition various so-called toughening constituents such as nickel, chromium, vanadium and the like. It has also been customary to incorporate various carbide-forming elements such as molybdenum,

the carbon content of the alloy may be increased above 20% to as high as the eutectoid percentage without introducing deleterious amounts of internal stress and strain in the hardened object and, quite to the contrary, I may obtain thereby a case-hardened article having a core evidencing very much higher load-carrying capacities than heretofore obtainable and a hardened area sub- To compenstantialiy free from the hardening cracks and fissures heretofore obtained in the practice of the prior art quench and temper method of hardening.

I have found that the steel composition for 5 case-hardening purpose, in addition to having the higher carbon (within the range above specified) must also be varied somewhat as to associated alloy constitutents with respect to the section of the object to thereby provide a composition sus- 10 ceptible to case hardening by the practice of the Bain et al. method.

The Bain et al. method of hardening briefly stated consists in causing austenite to undergo substantially complete transformation into stable ferritic structures at some temperature within the range of temperatures below that of rapid pearlite formation and above that of rapid martensite formation. In order to accomplish this, the steel first "must be heated to a temperature 20 above the critical range and converted fully into austenite and then the steel must be rapidly cooled to the temperature within this specified range at which transformation is to occur.

Withany given mode of cooling as the section increases the rate of heat transfer from interior to exterior decreases and consequently the time interval required to cool the major portion of the mass of the object to the desired temperature thereby increases.

As indicated in Fig. 2 of the Bain et a1. drawings, with any given alloy composition, for example, the plain carbon steel composition indicated, the time interval for transformation to initiate at temperatures below the upper critical temperature may vary widely from a few seconds to many hours depending upon the temperature. The time interval required to initiate the transformation of austenite into pearlite or into martensite is only a matter of seconds. 40

Therefore, a case-carburized object must be quenched at such a rate as will carry the temperature of the major mass of the metal to below the temperature of rapid pearlite formation in a relatively few seconds inorder to avoid the formation of relatively soft pearlite in the core metal of the casecarburized object. With objects of relatively heavy section this manifestly is impractical by any known means of rapid cooling. But I have found that a very expedient way of overcoming this dimculty is to incorporate in the plain carbon steel such a'proportion of a transformation retarding element or elements as will extend the time of initiation of transformation to such a period of time necessary with any given mode of cooling to insure that transformation will not. initiate until the temperature of the major mass of the article is below the temperature of rapid pearlite formation.

Retarding elements found satisfactory for this purpose are nickel, chromium, molybdenum, tungsten, vanadium. These may be utilized singly or in various combinations. For example, nickel in amounts ranging from 1 to 6% has been found very effective for this purpose; and nickelchromium in combination also has been found to be very effective, in which case nickel in amounts ranging from 1 to 4% and chromium in amounts ranging from 0.25 to 2% may be used. The specific amount of the transformation retarding ele- -ment incorporated in the alloy may be varied metal should be protected against pearlite forma- .tion.

In very heavy sections this may not be either desirable or necessary and accordingly the amount of the retarding agent incorporated need be only that required to insure the transformation of a desired mass of the core into ferritic structures other than pearlite and martensite at the quench temperature.

In association with the transformation retarding element I may utilize one or more of the elements (chromium, molybdenum, tungsten, vanadium) heretofore recognized in the art as benefiting the carburized area either as toughening or hardening agents. These elements in part singly or in combination with or without nickel or manganese also function as retarding elements as hereinabove discussed. Accordingly, in the broadest aspect the present invention comprises a casecarburizing steel consisting (1) of a plain carbon steel having a carbon content above .20% and not in excess of the eutectoid percentage; of the same steel plus a proportion of a transformation retarding agent or agents adapted with any given mode of cooling to give a time interval of initiation of transformation at least sufiicient to permit cooling of the major mass of the steel to below the temperature of rapid pearlite formation and the case-carburized area down to the desired hardening temperature, which hardening temperature preferably is within the lower temperatures of the range defined in the Bain et al. patent; and (3) either alloy composition of (1) and (2) above containing a proportion of an alloy constituent adapted tobenefit by hardening or toughening or both, the case-carburized areaof the steel.

As a specific embodiment of the practice of the present invention, a plain carbon steel (as defined in (1) above) having a carbon content above .20% and not in excess of the eutectoid percentage may be case carburized in accordance with prior art practice and may be case hardened by the practice of the method of the Bain -et al. patent, in sections up to thickness Without the formation of substantial amounts of pearlite in the center core member. As an example, square bars of plain carbon steel containing .64% carbon may be case hardened by quenching from the carburizing temperature down to about 400 F. in a molten salt bath and maintained at this temperature for a period of about 5 hours. Such a case-hardened steel will evidence a maximum outer fibre stress of about 312,000 pounds per square inch as compared to about 68,000 pounds per square inch when the same steel is quenched and tempered in accordance with prior art practice. This indicates the relatively higher loadcarrying capacity of the case-hardened steel of the present invention over that obtainable by prior practice.

As a comparison, the load-carrying capacity of this 54% carbon steel hardened by the Bain et al. method was over twice that of a typical low carbon nickel-chrome case-hardening steel of the same section hardened by the quench and temper method.

In addition to this advantage of higher loadcarrying capacity, the hardened case by reason of the fact that martensite formation is avoided in the thermal hardening method, is superior physically to the case obtained by the quench and temper method. The superiority is evidenced.

by the fact that shallower cases may be employed and that re-grinding costs are reduced to a minimum.

Where the section and size of the article is in excess of. that permitting relatively rapid quenching of the article down to the desired hardening temperature (for example,. 400 F.) various alloying percentages of manganese, nickel, chromium or nickel and chromium, manganese and chromium, may be incorporated in the plain carbon steel to retard the transformation of austenite into ferrite for a sufiicient time interval to permit such cooling.

For example, nickel about 3.5% will retard transformation about five (5) seconds, while nickel 1.25% and chromium .65% will retard transformation about forty (40) seconds. With this latter combination (Ni.Cr) I am enabled to successfully case harden objects having a thickness of one inch at 400 F. as above described and prevent the core metal from undergoing transformation into pearlite obtaining instead ferritic structures stable at temperatures intermediate the temperatures of rapid martensite and rapid pearlite formation but higher than v Having broadly and specifically described the present invention and given several specific embodiments thereof, it is apparent that many modifications may be made without departing essentially from the'nature and scope of the invention as. may

claims.

What I claim is:

1. The method of hardening a steel article comprised of carbon steel containing the usual amounts of Mn, Si, S and P and containing carbon in excess of .20% but not more than the eutectoid percentage, the marginal areas of said article containing a higher carbon content than the central or core area of the article, which comprises rapidly cooling the article from a temperature above the upper critical temperature at which the said steel is fully austenitic to a temperature below the temperature of rapid pearlite formation but above the temperature of rapid martensite formation and holding the said article at a temperature within the lower portion of this said range until theaustenite has undergone substantially complete transformation into ferritic structures other than martensite which are stable at the said holding temperature, and then cooling to atmospheric temperatures.

2. The method of claim 1, the said holding temperature approximating but above about 400 F.

3. The method of claim 1, the said rapid cool- I ing being at a rate adapted to lower the temperature of the major proportion of the mass of the article to below the temperature of rapid pearlite formation before transformation of the austenite into ferritic structures initiates.

4. The method of claim 1, in which the said steel composition is modified by incorporating within the carbon steel, prior to forming the said article, a proportion of a transformation retarding element consisting of at least .one of the elements Mn, Cr, Ni, W, Mo and V in an amount sufllcient to retard the initiation of transformation during rapid cooling for a time interval adapted to permit the cooling of the greater proportion of the mass of the article to temperatures below the temperature of rapid pearlite formation with the mode of cooling entployed.

5. In the case hardening of steel containing carbon in excess of 20% but not over the eutectoid percentage, the method which comprises case carburizing the said steel at a temperature above the upper critical temperature and rapidly cooling the case-carburized product from the carburizing temperature to a temperature approximating but above about 400 F. and holding the said steel at this said temperature for a time interval adapted to permit the said steel to undergo transformation from austenite into ferritic structures other than martensite which are stable at the temperature of holding, and then cooling the steel to atmospheric temperatures.

6. In the case hardening of steel containing carbon in excess of .20% butnot over the eutectoid percentage and a proportion of a transformation retarding element of the group consisting of Mn, Cr, Ni, W, Mo and V, which comprises case carburizing the steel at a temperature above the upper critical temperature, coolai acpai ing the article from the carburizing temperature to a temperature below the temperature of rapid pearlite formation at a rate sufiicient to cool the major proportion of the mass of the steel to below the temperature of rapid pearlite formation before transformation initiates, interrupting the cooling of the steel before any portion of the masshas cooled to below about 400 F., and holding the temperature of the steel at temperatures approximating but above 400 F. for a time interval adapted to permit substantially complete conversion of the austenite into ferritic structures stable at the holding temperatures, and then cooling to atmospheric temperatures.

7. An article of manufacture comprised of steel containing the usual percentages of Mn;

Si, S and P common to case-hardening steel but containing carbon in the core metal in amounts ranging from above 20% to the eutectoid percentage, the said article having been case carburized and hardened by the method of claim 1.

8. An article of manufacture comprised of steel containing the usual percentages of Si, S and P common tocase-hardening steel but containing carbon above 20% but not over the eutectoid percentage and a proportion of a transformation retarding agent consisting of at least one of the metals of the group Mn, Cr, Ni, W, Mo and V, in anamount adapted to retard the initiation of transformation of austenite into pearlite for a time interval sufficient to permit the cooling of the major portion of the mass of the article from a temperature above the upper critical temperature to a temperature below the temperature of rapid pearlite formation, the said article having been case carburized and hardened by the method of claim 5.

BERNARD R. QUENEAU. 40 

